This invention relates to chemical reactors for the conversion of a reaction fluid while replacing catalyst and indirectly exchanging heat with a heat exchange fluid.
In many industries, like the petrochemical and chemical industries for instance, the processes employ reactors in which chemical reactions are effected in the components of one or more reaction fluids under given temperature and pressure conditions. Most of these reactions generate or absorb heat to various extents and are, therefore, exothermic or endothermic. The heating or cooling effects associated with exothermic or endothermic reactions can positively or negatively affect the operation of the reaction zone. The negative effects can include, among other things, lower product production, deactivation of the catalyst, production of unwanted by-products and, in extreme cases, damage to the reaction vessel and associated piping. More typically, the undesired effects associated with temperature changes will reduce the selectivity or yield of products from the reaction zone.
One solution to the problem of negative temperature effects has been the indirect heating of reactants and/or catalysts within a reaction zone with a heating or cooling medium. The most well known catalytic reactors of this type are tubular arrangements that have fixed or moving catalyst beds. The geometry of tubular reactors poses layout constraints that require large reactors or limit throughput.
Indirect heat exchange has also been accomplished using thin plates to define alternate channels that retain catalyst and reactants in one set of channels and a heat transfer fluid in alternate channels for indirectly heating or cooling the reactants and catalysts. Heat exchange plates in these indirect heat exchange reactors can be flat or curved and may have surface variations such as corrugations to increase heat transfer between the heat transfer fluids and the reactants and catalysts. Although the thin heat transfer plates can, to some extent, compensate for the changes in temperature induced by the heat of reaction, not all indirect heat transfer arrangements are able to offer the complete temperature control that would benefit many processes by maintaining a desired temperature profile through a reaction zone. Many hydrocarbon conversion process will operate more advantageously by maintaining a temperature profile that differs from that created by the heat of reaction. In many reactions, the most beneficial temperature profile will be obtained by substantially isothermal conditions. In some cases, a temperature profile directionally opposite to the temperature changes associated with the heat of reaction will provide the most beneficial conditions. An example of such a case is in dehydrogenation reactions wherein the selectivity and conversion of the endothermic process are improved by having a rising temperature profile that reverses the normal adiabatic temperature gradient through the reaction zone. A specific arrangement for heat transfer and reactant channels that offers more complete control can be found in U.S. Pat. No. 5,525,311 B1, the contents of which are hereby incorporated by reference.
Most catalysts for the reaction of hydrocarbons are susceptible to deactivation over time. Deactivation will typically occur because of an accumulation of deposits that cause deactivation by blocking active pore sites or catalytic sites on the catalyst surface. Where the accumulation of coke deposits causes the deactivation, reconditioning the catalyst to remove coke deposits restores the activity of the catalyst. Coke is normally removed from the catalyst by contact of the coke-containing catalyst with an oxygen-containing gas at a high enough temperature to combust or remove the coke in a regeneration process. The regeneration process can be carried out in situ or the catalyst may be removed from a vessel in which the hydrocarbon conversion takes place and transported to a separate regeneration zone for coke removal. Arrangements for continuously or semi-continuously removing catalyst particles from a bed in a reaction zone for coke removal in a regeneration zone are well known. U.S. Pat. No. 3,652,231 B1 describes a continuous catalyst regeneration process which is used in conjunction with the catalytic reforming of hydrocarbons, the teachings of which are hereby incorporated by reference. In the reaction zone of U.S. Pat. No. 3,652,231 B1, the catalyst is transferred under gravity flow by removing catalyst from the bottom of the reaction zone and adding catalyst to the top while reactants flow cross-currently through a radial flow bed. U.S. Pat. No. 5,073,352 B1 describes a reforming reaction system adapted to move a compact bed of catalyst downward in reactant channels for on-stream catalyst replacement while a reactant stream flows cross-wise through the reactant channels and while heat exchange channels, interleaved with the reactant channels, provide indirect heat from a heating stream. It is also known from U.S. Pat. No. 2,550,727 B1 to move catalyst through a compact bed of catalyst particles by withdrawing particles from the compact bed of catalyst particles while passing reactants through the compact bed of catalyst in a direction co-current to the direction of catalyst movement.
A phenomenon known as xe2x80x9cpinningxe2x80x9d inhibits catalyst transfer in many reactor arrangements. Pinning is the phenomenon wherein the flow of fluid at sufficient velocity can block the downward movement of catalyst. Pinning is a function of the gas composition, the gas velocity, the physical characteristics of the catalyst and the physical characteristics of the flow channel through which the catalyst must move. As the gas flows through the channels that retain the catalyst, the gas impacts the catalyst particles and raises intergranular friction between the particles. When the vertical component of the frictional forces between the particles overcomes the force of gravity on the particles, the particles become pinned. As the flow path length of gas through the catalyst particles becomes longer, the forces on the particles progressively increase from the outlet to the inlet of the flow channel. In addition, as the catalyst flow channel becomes more confined, the gravity flow of catalyst particles becomes more hindered. Accordingly, as the size of the flow channel becomes more confined, wall effects increasingly add to the vertical hold-up force on the catalyst particles. As a result, narrow flow channels have a greater susceptibility to pinning and cannot normally provide continuous catalyst circulation.
In the case of reactors providing indirect heat exchange, the arrangement of the reactor exacerbates the problem of catalyst pinning. Increasing the number of channels by decreasing their size facilitates heat transfer by increasing the surface area between the heat exchange fluid and the catalyst. In addition, heat transfer is further facilitated by irregularities in the plate surface that create turbulence and reduce film factors that interfere with heat exchange. However, irregularities in the plates that define the channels further interfere with the movement of catalyst and promote a greater tendency for the catalyst to xe2x80x9cpinxe2x80x9d. Therefore, methods and reactor arrangements are sought to use a channel-type reactor that facilitates heat exchange and catalyst circulation while the reactor continues operation.
It is known to avoid some of the complexities of on-stream catalyst circulation by using a combination of fixed bed and moving bed reactor systems in series to treat a reactant stream. Combinations of series flow reactant streams that pass reactants through a fixed bed upstream of a moving bed system, downstream of a moving bed system, or have parallel trains of moving bed and fixed bed systems are shown in U.S. Pat. No. 5,417,843 B1, U.S. Pat. No. 5,196,110 B1, U.S. Pat. No. 5,190,639 B1, U.S. Pat. No. 5,190,638 B1 and U.S. Pat. No. 4,849,092 B1. None of these references discusses the possibility of incorporating heat exchange into any of the reaction arrangements discussed therein.
Accordingly, it is an object of this invention to provide a process for the contact of reactants with a bed of catalyst while providing indirect heat exchange with a heat exchange fluid and on-stream circulation of the catalyst.
It is a further object of this invention to provide a simple reactor apparatus for indirect heat exchange of a reactant stream and for contact of the reactant stream with a bed of catalyst while obtaining advantages that attend on-stream circulation of the catalyst.
This invention is a process and apparatus for serial flow of a reactant stream through one reaction zone that performs simultaneous indirect heat exchange between the reactant stream and a heat exchange fluid and through another reaction zone in which catalyst can be at least intermittently moved while the reactants contact the catalyst. The process simplifies an arrangement for indirectly exchanging heat with a heat transfer fluid during the reaction of reactants and for replacing catalysts during the operation of a process where catalyst becomes quickly deactivated. The process arrangement of this invention is most beneficially used where the reactants benefit from heat exchange in a reaction zone that at the same time has a relatively low rate of catalyst deactivation compared to the other reaction zone in the series. While the invention is most beneficially used with a fixed bed of catalyst in the reaction zone that provides heat exchange, process simplification is still possible by the use of this invention to provide only a series of reaction zones that all have on-stream catalyst replacement and only one reaction zone in the series also provides indirect heat exchange.
A number of endothermic reactions are advantageously carried out using the process and arrangement of this invention. In the dehydrogenation of light hydrocarbons, significant amounts of potential product are lost through thermal decomposition as reactants are heated to enter the first reactor. It has been found that up to 70% of the thermal cracking losses in a dehydrogenation reactor arrangement are attributable to thermal losses associated with the heat up and conversion of reactants in the first reaction of the series. It has also been found that the first reaction zone in the series will have an overall coking rate that is lower than the subsequent reactors in the series and relatively low overall. In accordance with this invention, the process can be substantially improved by the use of a fixed bed reaction zone that performs simultaneously heat exchange to heat the reactants in the reaction zone and to substantially reduce losses due to thermal cracking. Similar process characteristics can contribute to beneficial operations of reforming processes using a fixed bed heat exchange reactor for the first reaction zone. Another reaction that may benefit from the process arrangement of this invention is the production of styrene by dehydrogenation.
A combination of catalyst replacement and indirect heat exchange with a heat transfer fluid can also provide a reaction advantage for processes that use this invention. This combination can provide an isokinetic reaction condition within the reaction stacks. As catalyst is incrementally replaced in the reaction stacks, the most deactivated catalyst is removed from the bottom while the most active catalyst enters the top of the reaction stack. This periodic replacement thereby provides a continuous activity gradient down the length of the catalyst bed in each reaction stack. The decrease in activity can be compensated for by an increase in the reaction temperature. In the case of an endothermic reaction where a heating fluid enters the heat transfer channels, the fluid can enter the heat transfer channels in a flow direction that compensates for the loss of activity in the catalyst. By passing the heat exchange fluid from the bottom of the reaction stack to the top of the reaction stack, higher temperatures are maintained in the lower portion where the more deactivated catalyst contacts reactants. Progressing upwardly through the reaction stack, heating of the reactants cools the heating medium thereby resulting in a relatively reduced temperature for the reactants in the upper portion of the reaction stack which contains the most active catalyst. Tailoring of catalyst replacement, heating medium temperature and heat exchange across the reactant and heat exchange channels can be arranged to provide an isokinetic operation across the reaction zone. This isokinetic operation can result in a more uniform product effluent and the most efficient utilization of the reaction volume in each reaction stack. Isokinetic conditions can be maintained with exothermic reactions as well as with endothermic reactions. In exothermic reactions, the cooling medium should enter the top of the heat exchange channels to maintain a co-current flow with the catalyst so that the maximum cooling is provided at the region of the most active catalyst.
Accordingly, in a broad embodiment, this invention is a process for catalytically converting a feed comprising at least one reactant in an endothermic or exothermic reaction to a reacted stream comprising conversion products by passing the feed serially through at least two reaction zones. The process passes the feed serially through a least one heat exchange reaction zone and at least one moving bed reaction zone in any order. The feed or an effluent from a moving bed reaction zone contacts catalyst at conversion conditions in the heat exchange reaction zone to effect conversion of reactants while simultaneously performing an indirect heat exchange between a heat exchange fluid and the catalyst and reactants to produce a heat exchange reaction zone effluent. The feed or the heat exchange reaction zone effluent contacts a second catalyst comprising catalyst particles at conversion conditions in the moving bed reaction zone to produce a moving bed reaction zone effluent. The process at least periodically withdraws catalyst from the bottom of the moving bed reaction zone and adds catalyst to the top of the moving bed reaction zone while the feed or the heat exchange reaction zone effluent passes through the moving bed reaction zone. The process withdraws a product stream comprising conversion products that have passed through the heat exchange reaction zone and the moving bed reaction zones.
In a more limited embodiment, this invention is a process for catalytically converting a feed comprising hydrocarbons in an endothermic or an exothermic process by passing the feed through at least two reaction zones. The process passes the feed to a first reaction zone and contacts the feed with a catalyst at hydrocarbon conversion conditions to effect hydrocarbon conversion to produce a first reaction zone effluent in the first reaction zone while simultaneously performing an indirect heat exchange in the first reaction zone between a heat exchange fluid and the catalyst and reactants. The first reaction zone effluent passes to a second reaction zone and contacts the second reaction zone effluent with a second catalyst comprising catalyst particles to effect a further conversion of hydrocarbons in the first reaction zone effluent. The process at least periodically withdraws catalyst from the bottom of the second reaction zone and adds catalyst to the top of the second reaction zone while the first reaction zone effluent passes through the second reaction zone. The process withdraws a product stream from the process comprising converted hydrocarbons that have passed through the first and second reaction zones.
In an apparatus embodiment, the invention is a reactor arrangement for contacting reactants with catalyst and indirectly heat exchanging the reactants with a heat transfer fluid and replacing catalyst particles on stream. The arrangement includes a first reactor vessel defining a feed inlet for the addition of a reactant thereto, defining an intermediate outlet for the withdrawal of an intermediate stream therefrom, containing a heat exchange surface that defines a plurality of reaction channels on one side of the heat exchange surface for retaining a catalyst and defines a plurality of heat exchange channels on an opposite side of the heat exchange surface for circulation of heat exchange fluid therein. A second reactor vessel defines an intermediate inlet for adding at least a portion of the intermediate stream thereto, defines a reacted outlet for withdrawing a reacted stream therefrom, contains a perforated surface for retaining catalyst particles in a contacting bed, defines a particle addition inlet for adding catalyst particles to the contacting bed and defines a particle withdrawal outlet for removing catalyst particles from the contacting bed. The apparatus also includes a conduit for communicating the intermediate outlet with the intermediate inlet.
Additional embodiments, arrangements and details of this invention are disclosed in the following detailed description of the invention.